It must be a feature then?
I think the situation is only possible if the giant stands on a tile with road, so he is able to move 2 tiles, but the pathfinder algorithm decides that he can jump on a ship, and disembark in the same turn and cause splash damage?
If the same giant stands in front of a port like so
L-W-L (Land water land) G-P-L1 (giant, port, land) L-W-L, He would not be able to cross the river in one turn to tile L1 right?
A giant, or any other unit, should not be able to embark and disembark on the same turn.
If they can, that's a bug